1. Field of the Invention
The present invention relates to an anode having an anode active material layer capable of inserting and extracting an electrode reactant on an anode current collector and a secondary battery using the same.
2. Description of the Related Art
In recent years, portable electronic devices such as a video camera, a digital still camera, a mobile phone, and a notebook personal computer have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as an electric power source for the portable electronic devices, a battery, in particular, a small and light-weight secondary battery capable of providing a high energy density has been developed.
Specially, a lithium ion secondary battery using insertion and extraction of lithium ions for charge and discharge reaction is in practical use widely, since such a lithium ion secondary battery is able to provide a higher energy density than a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes a cathode containing a cathode active material capable of inserting and extracting lithium ions, an anode containing an anode active material capable of inserting and extracting lithium ions, and an electrolyte.
As the anode active material, a carbon material is widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improving the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
In the case where silicon is used as an anode active material, as a method of forming the anode active material layer, vapor-phase deposition method such as evaporation method is used. In this case, silicon is directly deposited on the surface of the anode current collector, and the anode active material is linked to (fixed on) the anode current collector, and thus the anode active material layer is hardly expanded and shrunk at the time of charge and discharge.
However, in this case, there is concern that cycle characteristics and swollenness characteristics as important characteristics of the secondary battery are lowered for the following reasons.
Firstly, at the time of charge and discharge, the anode active material inserting lithium ions becomes highly active. Thus, the electrolyte is easily decomposed, and part of the lithium ions easily becomes inert. Thereby, after charge and discharge are repeated, the discharge capacity is easily lowered, and gas is easily generated in the battery due to decomposition of the electrolyte.
Secondly, even if the anode active material is linked to the anode current collector, there is a possibility that the anode active material layer is broken and dropped from the anode current collector depending on the degree of expansion and shrinkage of the anode active material layer at the time of charge and discharge. Thereby, after charge and discharge are repeated, the discharge capacity is easily lowered. Further, if the anode active material layer is intensely expanded and shrunk, consequently the anode current collector becomes easily deformed.
Thirdly, since the silicon depositional film becomes amorphous (amorphous), the anode active material is easily affected by oxidation. Thus, the physical property of the anode active material is deteriorated with age, and the contact strength of the anode active material layer to the anode current collector is easily lowered. Thereby, after charge and discharge are repeated, the discharge capacity is easily lowered.
Therefore, to improve the cycle characteristics and the like of the lithium ion secondary battery, several technologies have been proposed. Specifically, the anode active material is formed to contain a crystalline region and an amorphous region (for example, refer to WO 01/029912). Further, a plurality of anode active material particles containing silicon and lithium are used and are bound by being sintered or fused (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-269331). Further, the anode active material layer is formed so that a lower layer having primary particles grown by using vapor-phase deposition method and an upper layer having secondary particles deposited by using coating method are layered (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-122915).